Fix assignability of unconstrained T[K]

[Anders Hejlsberg (ahejlsberg)]

This PR changes the assignability of an unconstrained indexed access to be the same as that of an unconstrained type parameter. Specifically, an unconstrained type parameter is assignable to {} and with this PR the same is true for an unconstrained indexed access.

function foo<T, K extends keyof T>(x: T, y: T[keyof T], z: T[K]) {
    let a: {} | null | undefined;
    a = x;  // Ok
    a = y;  // Ok, but was error before this PR
    a = z;  // Ok, but was error before this PR
}

Fixes #21368.
Fixes #23133.